Rename ClosureKind variants and stop re-exporting them.

[Ms2ger]

No description provided.

[rust-highfive]

r? @brson

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[oli-obk]

the commit message needs a [breaking-change]

[nikomatsakis]

I'm not convinced this sort of change out to be tagged with [breaking-change]. It's one thing to tag changes to libsyntax which (unfortunately, in my view) has a kind of "semi-stable" status, but the internals of rustc don't fall in that category.

[oli-obk]

it's reachable from late lints, I just assumed that any change observable from plugins would require the mark so we can find the commits easily

[Ms2ger]

I can see the argument for including it, but I haven't done that with similar changes I made earlier. Happy to go either way.

[alexcrichton]

r? @eddyb (or anyone else from the compilers team)

[eddyb]

Pretty! @bors r+

[bors]

📌 Commit c6474af has been approved by eddyb

[bors]

⌛ Testing commit c6474af with merge a9ffe67...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt